Magnetization Manipulation of a Flexible Magnetic Sensor by Controlled Stress Application

Abstract

Spin-based electronic devices on polymer substrates have been intensively investigated because of several advantages in terms of weight, thickness, and flexibility, compared to rigid substrates. So far, most studies have focused on maintaining the functionality of devices with minimum degradation against mechanical deformation, as induced by stretching and bending of flexible devices. Here, we applied repetitive bending stress on a flexible magnetic layer and a spin-valve structure composed of Ta/NiFe/CoFe/Cu/Ni/IrMn/Ta on a polyimide (PI) substrate. It is found that the anisotropy can be enhanced or weakened depending upon the magnetostrictive properties under stress. In the flat state after bending, due to residual compressive stress, the magnetic anisotropy of the positive magnetostrictive free layer is weakened while that of the pinned layer with negative magnetostriction is enhanced. Thus, the magnetic configuration of the spin-valve is appropriate for use as a sensor. Through the bending process, we design a prototype magnetic sensor cell array and successfully show a sensing capability by detecting magnetic microbeads. This attempt demonstrates that appropriate control of stress, induced by repetitive bending of flexible magnetic layers, can be effectively used to modify the magnetic configurations for the magnetic sensor.

Figure 1

(a) and (b) Hysteresis loops for NiFe and Ni layers with 4-nm thickness in the initial-flat-state (black square symbols), bent-state (purple circle symbols), and in the flat state after bending (after-flat-state), (orange triangle symbols). Inset of (b) shows the full magnetization curve for the Ni layer in the bent-state. (c) The magnetic layer on the PI substrate in the downward bent state with the smallest bending radius of 3.9 mm. The black arrow indicates the magnetic easy axis on the back side of the PI substrate.

Figure 2

(a) The flexible spin-valve film in the bending machine in the initial-flat-state, bent-state, and in the flat state after bending (after-flat-state) from top to bottom. (b) Optical images for the top surface of the spin-valve. b-1 is the initial surface, and b-2 to b-6 are the surfaces after 5 (red), 25 (green), 50 (blue), 100 (cyan), and 200-times (pink) bending applied sequentially. Arrows indicate the location of cracks produced after each step. (c) Magnetization curves for the flexible spin-valve in the after-flat-state for varying numbers of bending cycles. (d) and (e) Expanded plots of (c) near zero and −250 Oe, respectively, in the initial-flat-state and in the after-flat-state for 200-times bending.

Figure 3

Schematic illustration of magnetic configuration in a spin-valve on flexible PI substrate before and after downward bending stress is applied along the direction of the magnetic easy axis.

Figure 4

(a) and (b) Full and minor MR ratio loops in the flat state after 5, 25, 50, and 100-times repetitive bending, respectively, for current flows along the cracks (solid symbols). Insets of (a) and (b) show the change in 2H_c for the pinned layer (yellow symbols) and 2H_k of the free layer (brown symbols) in terms of the number of bending cycles, respectively. (c) Change in the MR ratio (purple symbols), R_min (magenta symbols), and sensitivity (turquoise symbols) as the number of bending cycles increases. (d) and (e) Full and minor MR ratio loops in the flat state after 5, 25, 50, 100, and 200-times repetitive bending, respectively, for current flow across the cracks (open symbols). Insets of (d) and (e) also show the change in 2H_c and 2H_k for the pinned (yellow symbols) and free (brown symbols) layers, respectively, depending on the number of repetitive bending cycles. (f) Change in the MR ratio (purple symbols), R_min (magenta symbols), and sensitivity (turquoise symbols) with respect to number of repetitive bending cycles.

Figure 5

(a) Optical images for a spin-valve sensor cell array on a PI substrate. Magnified image on the right side shows a central part of the sensor cell array. The large yellow horizontal rectangles and small green squares are the contact pads and cells, respectively. (b) Optical images of the reference cells and sensing cells, with a window for placement of magnetic microbeads onto the centre of the array. The image on the right side shows an enlarged sensing cell with cracks after 50-times bending process. (c) SEM images for the 2.8-μm magnetic microbeads on the surface of the sensing cells. Arrows indicate the directions of external sweeping and modulation fields. (d) Minor MR ratio loops for the reference cells (black square symbols) and sensing cells (red circle symbols) before bending (open symbols) and after 50-times bending (solid symbols). (e) Voltage changes for the reference and sensing cells at zero sweeping field along the x-axis. The symbols in the grey and white background areas indicate data in the initial state before bending and in the flat state after 50-times bending, respectively. (f) Normalized MR ratio loops for sensing cells before (red circle symbols) and after (green circle symbols) introduction of the magnetic microbeads. Enlarged parts clearly show a shift in the measured loops.